1. Field of the Invention
This invention relates generally to a tracking servo system and, more particularly, to a tracking servo system for use with an optical pick-up that extracts an information signal from an optical record medium.
2. Description of the Prior Art
Information reproduction systems are now becoming well known in which the information medium is a disc having the information arranged thereon as an array of pits in the disc surface. The information on the disc is read by a light beam and, thus, is generally known as an optical disc. Typically, the light beams are emitted from a semiconductor laser source and focussed to a small spot size in accord with the quite small size of the pits formed in the optical disc. The disc is rotated and the light beam caused to scan the surface, with the modulated reflections of the light beam from the disc then being received by a photo-detector and processed to reproduce the desired information signal. The pits are generally formed having a depth which is some fraction of the wavelength of the light beam and, typically, the pit depth is chosen as one-fifth, one-fourth, or one-third the wavelength of the monochromatic light beam from the laser.
It is known to generate servo control signals to control tracking or scanning of the light beam by using the photo-detector upon which the reflected light forms a diffraction pattern and which provides the reproduced information signal. Using known servo-control techniques, a tracking error signal is used to drive the light beam into alignment with the pits on the record medium surface. One approach in generating the tracking error signal is to divide the photodetector into individual light receiving areas and then to take off the signals from each area in various combinations to produce a differential tracking error signal. This approach to producing a tracking error signal is generally referred to as differential phase detection (DPD). Typically, the diffraction pattern formed on the photo-detector is symmetric with the center of the coordinates of the multiple light-receiving areas. Problems arise in this tracking error system when the depth of the pits in the disc is changed from one-fourth wavelength to, for example, one-fifth or one-third the wavelength of the scanning light beam, which is evidenced by the diffraction pattern losing symmetry along a line dividing the light-receiving areas in a direction normal to the tracking direction. This results in a DC component that is included in the tracking error signal as a difference between selected pairs of receiving areas of the photo-detector used in DPD tracking. The magnitude of this DC component then changes as the diffraction pattern moves over the various areas of the light-receiving surface of the photo- detector. This DC component that is present when the pit depth is one-fifth of a wavelength is particularly troublesome during a random access operation, where the light beam jumps over several hundreds of tracks to a specific track. In that case, when the tracking error signal has a DC level at the start of the "read" time portion of the tracking control signal, false tracking errors are caused that result in unsteady operation and improper jumping.
Another situation in which the DC component in the tracking error signal is problematic is when a strong external shock is imparted to the tracking servo device, in which case the tracking servo device tends to oscillate at a specific resonant frequency. This comes about because in this kind of optical disc system, even though the tolerance of the positional deviation of the information tracks relative to the center of the optical disc during the manufacturing process, or at the time of "cutting" the optical disc, is held within a range of about 130 microns, this small amount of error causes an increase in the tracking error when the low-frequency gain is low. In order to prevent this, the low-frequency gain of the servo system is generally increased and appropriate phase compensation is provided. Thus, when a DC component is present in the tracking error signal, as might occur in the above situations, the conditions for oscillation (and instability) are established when the random access mode is being used or when a strong external shock acts on the system. Such conditions for oscillation may be typified by an open-loop transfer function gain of the servo system of unity and a phase shift of 180.degree. at the resonant frequency of the oscillation. This results in the tracking servo system commencing to oscillate and, in such case, oscillation continues and the tracking servo signal has a waveform with a substantially discernible resonant frequency.